1. Field of the Invention
This invention relates generally to an apparatus and method for sensing physical phenomena and particularly to an optical position sensing system for detecting the position of one or more displaceable elements. Still more particularly, the invention relates to a high accuracy light radar, fiber optic position sensing system for use on an aircraft in order to measure positions of various moving parts of the aircraft at high rates and with short lag times.
2. Background of the Related Art
Traditionally, electrical sensors are used to measure the position of various actuators in an aircraft which are used, for example, to control the position of various aerodynamic surfaces, such as flaps, rudder, ailerons, etc. Results of these measurements are then fed back to a system flight controller which processes this information and outputs appropriate commands to control the actuators.
A typical actuator has a rod secured within an outer casing. Depending on the actuator, the rod can move back and forth a maximum distance of a few millimeters to over 50 cms. This maximum distance is often referred to as a stroke. A sensor head associated with the actuator sends a position signal representing the position of the actuator rod to a processor that calculates a position measurement. Position measurements of the rod must be fed to the flight controller at rates up to several hundred Hz, with a lag time less than 0.5 ms, and accuracies of a few hundred micrometers. Here, lag time is defined to be the time between completion of raw data collection from a sensor and transmission of a position measurement to the flight controller.
Fiber optic position sensing systems offer numerous advantages over conventional electrical sensing systems. First, they are small and lightweight. In addition, they can be made immune from electromagnetic interference (EMI) which can occur near power lines, and electromagnetic pulses (EMP) which can occur in the event of a nuclear explosion. EMI/EMP immunity is an especially important advantage for new generation aircraft which have skins made largely of composite (non-metallic, non-shielding) material. Without heavy, bulky and expensive shielding of conventional electrical sensors and control lines, these next generation aircraft can not be safely flown in areas of severe EMI/EMP. Therefore, "fly-by-light" systems or fiber optic position sensing systems have the potential to replace "fly-by-wire" systems in future aircraft.
Some fiber optic position sensing systems use digital or optical encoding techniques in order to vary the amplitude of an incident optical signal as a surface is moved. However, sensor heads for these types of sensor systems cannot be easily multiplexed and consequently cost, complexity, weight and volume of the system are increased.
Another type of fiber optic position sensor system sometimes called an optical time domain reflectometer (OTDR) uses a pulsed optical source. In particular, OTDRs measure distances to in-line fiber reflectors by estimating a round trip transit time of a light pulse from the pulsed optical source to the in-line fiber reflector and back to a detector. Both the measurement accuracy and estimation times are fundamentally limited by the amplitude and width of the light pulse. Consequently, conventional OTDRs are usually only capable of position measurement accuracies of 5 to 500 centimeters. Some OTDRs measure distances to a Fresnel reflector at a fiber end face with submillimeter accuracy using pulse widths of 50 to 100 pS. However, these OTDRs require estimation times which are orders of magnitude longer than the submillisecond lag times T.sub.Li and cannot achieve the several hundred Hz update rates R.sub.i required for aircraft position sensing applications. Finally, it can be difficult to multiplex multiple sensor heads in OTDR systems.
Still another type of fiber optic position sensor system is a coherent optical frequency domain reflectometer (COFDR). COFDRs use coherent frequency modulated (FM) optical radiation. However, optical sources used in the COFDR must have narrow line widths and therefore tend to have low output power and low reliability. Moreover, all fibers used in COFDRs must be single mode polarization preserving fibers in order to coherently optically mix returned FM optical signals with an optical local oscillator signal and consequently are difficult to install and maintain.
Another drawback of coherent FM systems is due to the large Doppler shifts which can occur. For example, a rod in an actuator can have velocities ranging up to several centimeters per second. Passive sensor heads measure the position of the rod by reflecting or scattering optical radiation from an end face of the rod. Hence, the frequency of this scattered optical radiation is Doppler shifted. Since the Doppler shift is proportional to the frequency of the optical radiation, it can be very large. For example, if the rod is moving at two centimeters per second, then the Doppler shift for optical radiation having a wavelength of 1.06 micrometers is about 38 kHz which can result in significant errors in position measurement.
One way to eliminate such errors is to utilize first an up and then a down chirp and measure two different beat or intermediate (IF) frequencies. These two beat frequencies can then be subtracted from each other thereby cancelling out the Doppler shift. However, this requires wide bandwidth detectors and filters which reduces the signal-to-noise ratio of the measurements and consequently the accuracy of the measurements. In addition, this approach requires two time periods to obtain one position measurement.
Finally, a fourth approach to fiber optic position sensing systems with passive sensor heads involves incoherent optical frequency domain reflectometry (IOFDR). IOFDR is discussed, for example, in "Performance of Integrated Source/Detector Combinations for Smart Skins Incoherent Optical Frequency Domain Reflectometry (IOFDR) Distributed Fiber Optic Sensors," by W. B. Spillman, Jr., P. L. Fuhr and B. L. Anderson, SPIE Vol. 986, Fiber Optic Smart Structures and Skins (1988). An IOFDR is shown in FIG. 6a of the Spillman reference. There the IF signal appears at the output of a mixer. The frequency of this IF signal is proportional to the time delay introduced by a white cell. However, this system is not capable of outputting position measurements at rates R.sub.i of several hundred Hz, lag times T.sub.Li under 0.5 ms and position accuracies .delta.L.sub.Si of several hundred micrometers which is required for a flight controller to control flight of an aircraft.
Other attempts have been made to implement IOFDRs. For example, "Fiber Optic Sensor Multiplexing by FMAMCW," by R. Gallay et al., Appl. Phys. 21 (1989) discloses a sensor network with three sensors L.sub.1 -L.sub.3. However, this system is also incapable of update rates R.sub.i of several hundred Hz with lag times T.sub.Li under 0.5 ms or position measurement accuracies .delta.L.sub.Si of several hundred micrometers.
Therefore, IOFDRs or frequency modulated continuous wave (FM-CW) systems have thus far been limited in their estimation of a target's position.